int wprintf( const wchar_t* format, ... );

int wprintf( const wchar_t* restrict format, ... );

int fwprintf( FILE* stream, const wchar_t* format, ... );

int fwprintf( FILE* restrict stream,
              const wchar_t* restrict format, ... );

int swprintf( wchar_t* buffer, size_t bufsz,
              const wchar_t* format, ... );

int swprintf( wchar_t* restrict buffer, size_t bufsz,
              const wchar_t* restrict format, ... );

int wprintf_s( const wchar_t* restrict format, ... );

int fwprintf_s( FILE* restrict stream,
                const wchar_t* restrict format, ... );

int swprintf_s( wchar_t* restrict buffer, rsize_t bufsz,
                const wchar_t* restrict format, ... );

int snwprintf_s( wchar_t* restrict s, rsize_t n,
                 const wchar_t* restrict format, ... );

Loads the data from the given locations, converts them to wide string equivalents and writes the results to a variety of sinks.

Parameters

The format string consists of ordinary wide characters (except %), which are copied unchanged into the output stream, and conversion specifications. Each conversion specification has the following format:

• introductory % character.

• (optional) flags that modify the behavior of the conversion:

• -: the result of the conversion is left-justified within the field (by default it is right-justified).
• 0: applicable to floating-point and integer conversions only. Leading zeros are used to pad the field instead of space characters. This flag is ignored if - flag is present. For integer conversions, this flag is ignored if the precision is explicitly specified. For floating-point conversions, this flag is ignored if the value is NaN or infinity. Other conversions using this flag have undefined behavior.
• +: applicable to floating-point and signed integer conversions only. Non-negative numbers are prefixed with a plus character. This flag is ignored by other conversions.
• space: applicable to floating-point and signed integer conversions only. Non-negative numbers are prefixed with a space character. This flag is ignored if + flag is present. This flag is ignored by other conversions.
• #: applicable to floating-point and non-decimal integer conversions only. The alternative form of the conversion is performed. See below for the exact semantics. Other conversions using this flag have undefined behavior.

• (optional) integer value or *: specifies minimum field width. The result is padded with space characters (by default), if required, on the left when right-justified, or on the right if left-justified. In the case when * is used, the width is specified by an additional argument of type int, which appears before the argument to be converted and the argument supplying precision if one is supplied. If the value of the argument is negative, then the - flag is implied and the absolute value used for minimum field width.

• (optional) . followed by non-negative integer number or *: specifies the precision of the conversion. In the case when * is used, the precision is specified by an additional argument of type int, which appears before the argument to be converted, but after the argument supplying minimum field width if one is supplied. If the value of this argument is negative, it is ignored. If neither a number nor * is used, the precision is taken as zero. See below for the exact semantics.

• (optional) length modifier that specifies the size of the argument (in combination with the conversion format specifier, it specifies the type of the corresponding argument).

• conversion format specifier.

If a conversion specification is invalid, the behavior is undefined.

Non-numeric

• %: Writes % character. Has no argument. No modifiers supported, full conversion specification must be %%.
• p: Writes a pointer. Argument is a void*, char*, signed char*, or unsigned char*(since C23). Output format is implementation defined.
• c: Writes a character. Argument type depends on the length modifier:

• (none): Argument is an int, which is converted to wide character via btowc and copied to the output stream.
• l: Argument is a wint_t, which is converted to a wchar_t and copied to the output stream.

• s: Write a string. Argument is a pointer to the initial element of an array of characters. Argument type depends on the length modifier:

• (none): Argument is a char*, signed char*, unsigned char*. String is converted to a wide string via mbrtowc (with zero-initialized conversion state), and resulting characters copied to the output stream. For the precision modifier and %n specifier, the number of characters refers to the wide string, not the converted-from narrow string.
• l: Argument is a wchar_t*. Characters are copied to the output stream.

The precision specifies the maximum number of characters to write. Characters are copied until the maximum is reached or a null terminator is found. The null terminator does not contribute to the number of characters reported by %n.

Even though %c expects int argument, it is safe to pass a char because of the integer promotion that takes place when a variadic function is called.

Integer

Let SInt and UInt be respective types determined by the length specifier:

• h: short and unsigned short
• (none): int and unsigned
• l: long and unsigned long

Then:

• d/i: Writes a signed integer in decimal representation. Argument is an SInt.
• u: Writes an unsigned integer in decimal representation. Argument is an UInt.
• x/X: Writes an unsigned integer in hexadecimal representation. Argument is an UInt. The alternative form prefixes 0x if the argument is non-zero. If the format specifier is uppercase, the corresponding output will also be uppercase.

• o: Writes an unsigned integer in octal representation. Argument is an UInt. The alternative form increases the precision as much as is needed for the first digit written to be zero.
• n: Returns the number of characters written so far by this call to the function, including minimum field width padding. Argument is an SInt* to write the result to. Only supported modifier is length modifier.

The precision specifies the minimum number of digits to write, leading zeros will be written if necessary. If unspecified, a precision of 1 is used. If precision is 0 and the argument is 0, no digits will be written (the plus or space character is still written if the + or space flag is specified).

The correct conversion specifications for the fixed-width character types (int8_t, etc) are defined in the header <inttypes.h> (although PRIdMAX, PRIuMAX, etc is synonymous with %jd, %ju, etc).

The memory-writing conversion specifier %n is a common target of security exploits where format strings depend on user input and is not supported by the bounds-checked printf_s family of functions.

There is a sequence point after the action of each conversion specifier; this permits storing multiple %n results in the same variable or, as an edge case, printing a string modified by an earlier %n within the same call.

Floating-point

The argument type is determined by the length specifier:

• (none): double

• L: long double

Then:

• If the argument is infinity, then inf or infinity is written. Which one is used is implementation defined.
• If the argument is not-a-number, then nan or nan(char_sequence) is written. Which one is used is implementation defined.

Otherwise:

• f/F: Writes a floating-point in decimal notation [-]ddd.ddd. The precision specifies the number of decimal places, trailing zeros will be written if necessary. If unspecified, a precision of 6 is used.
• e/E: Writes a floating-point in decimal exponent notation [-]d.ddde±dd. If the exponent is less than 10, it is written with a leading zero. The precision specifies the number of decimal places, trailing zeros will be written if necessary. If unspecified, a precision of 6 is used.
• g/G: Writes a floating-point in decimal or decimal exponent notation, depending on the argument and precision.

Let P = max(0, precision - 1), or 5 if precision is unspecified.

Let EXP be the exponent written if %e were the specification.

• If -4 ≤ EXP ≤ P, then the argument is written as if the f/F format specifier was used with precision P - EXP.
• Otherwise, the argument is written as if the e/E format specifier was used with precision P.

The alternative form writes a decimal point even if precision is 0. If the format specifier is uppercase, the corresponding output will also be uppercase.

Return value

Notes

While narrow strings provide snprintf, which makes it possible to determine the required output buffer size, there is no equivalent for wide strings (until snwprintf_s)(since C11), and in order to determine the buffer size, the program may need to call swprintf, check the result value, and reallocate a larger buffer, trying again until successful.

snwprintf_s, unlike swprintf_s, will truncate the result to fit within the array pointed to by buffer, even though truncation is treated as an error by most bounds-checked functions.

Example

#include <locale.h>
#include <wchar.h>

int main(void)
{
    char narrow_str[] = "z\u00df\u6c34\U0001f34c";
                  // or "zß水🍌"
                  // or "\x7a\xc3\x9f\xe6\xb0\xb4\xf0\x9f\x8d\x8c";
    wchar_t warr[29]; // the expected string is 28 characters plus 1 null terminator
    setlocale(LC_ALL, "en_US.utf8");
    swprintf(warr, sizeof warr / sizeof* warr,
             L"Converted from UTF-8: '%s'", narrow_str);
    wprintf(L"%ls\n", warr);
}

Converted from UTF-8: 'zß水🍌'

References

See also